Single frequency fiber lasers are commonly known to produce laser light with a very high degree of frequency stability and a corresponding very low level of frequency noise. This has made this class of lasers particularly suited for a number of applications within interferometric and remote sensing where the frequency noise of the laser light is a limiting factor. Development within especially the field of geo-seismic sensing has called for even lower levels of frequency noise than are typically obtained from standard, non-conditioned fiber lasers. Some of these applications further require that the lasers can be tuned while at the same time they must exhibit very low frequency noise. It is known from numerous publications within the field of laser based spectroscopy, and especially from the field of laser trapping and cooling that frequency locking of the laser frequency to stable interferometers with high finesse can lead to significant reductions in laser frequency noise (prior art examples include [Tröbs et al], [Gréverie et al]). Laser frequency locking in the prior art is typically obtained using large size interferometers which are un-suited for integration with compact laser systems devised for field applications. On the other hand, compact, hermetically packaged, and fiber coupled interferometers for laser wavelength locking are well known from the field of optical telecommunications, where so-called wavelength lockers are used to maintain the laser frequency within a typical filter bandwidth e.g. in a dense wavelength division multiplexed optical communication system. Such telecom grade wavelength lockers are un-suited for the purpose of laser frequency noise reduction due to their low coefficient of finesse (also simply called low finesse), and further they cannot be tuned in wavelength or frequency. The prior art describes a frequency tunable telecom grade wavelength locker ([Tuominen et al]). However, this wavelength locker again has a very low finesse and is hence unsuited for laser frequency noise reduction.
The prior art further describes a frequency tunable external cavity diode laser incorporating thermally tunable etalons e.g. as in US2004/0101016 A1. It is well known in the art that extending the cavity length of e.g. diode lasers leads to a reduction in linewidth and similarly in the fundamental phase noise as defined by the Shawlow-Townes limit. This is typically obtained by coupling the output of the diode laser to an external cavity comprising an output coupler and a high reflectivity mirror. However, when extending the cavity length, the cavity free spectral range is reduced, and spectral filtering elements are typically needed to ensure single mode operation. These spectral filtering elements (or mode filters) are placed inside the extended cavity, such as is the case for the tunable etalons described in US2004/0101016 A1. In this document the thermal tunability of the etalons secures frequency tunability of the external cavity laser by effectively changing the optical path length of the cavity. However, while the etalon in this manner assists in obtaining single mode operation, its potential insensitivity to technical noise (such as can be obtained via a compact etalon design and hermetic sealing) is not utilized.
WO 03/005502 A2 discloses a similar example where a thermally tunable etalon is placed inside the laser and is likewise used to tune the wavelength of an external cavity diode laser.